Process for preparing rotors or stators of a turbomolecular pump

ABSTRACT

The invention relates to a process for preparing rotors or stators of a turbomolecular pump with rotor blades made of a specific aluminum alloy.

The invention relates to a process for preparing rotors or stators of a turbomolecular pump with rotor blades made of a specific aluminum alloy.

For the construction of rotors of a turbomolecular pump with rotor blades, aluminum has become established as a construction material, because this is the best way to combine the demand for as low a density as possible and a high strength and ready processability. Thus, from M. Wutz et al., Theorie und Praxis der Vakuumtechnik, 2nd Ed., 1982, Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden, pages 207/208, it has already been known to prepare the rotor or stator from especially selected aluminum alloys.

To be able to construct high performance pump rotors or stators, aluminum alloys having a high high-temperature strength are predominantly employed. Rotors made of such materials are usually produced by means of machining methods as described, for example, in DE 10210404 A1 or DE 29715035 U1, both included herein by reference. In particular, the shaping of the blade contour is time- and cost-intensive.

DE 101 03 230 A1 describes rotors in which part of the rotor blades has a back side having a convex design on the suction side and a concave design on the pressure side, or that at least part of the rotor blades has a front side having a concave design on the suction side and a convex design on the pressure side.

When high strength alloys are used, their low reshapability is to be considered. As a result, the complete shaping of solid bodies or disks must be effected by means of machining methods, and less expensive shaping methods by plastic reshaping, such as twisting, embossing or coining, cannot be employed.

Concretely, for the medium strength alloys, a combination of machining (turning, cutting) and/or thermal ablation methods (eroding) with methods of plastic reshaping (twisting) has become established as a more economical preparation method for rotors or stators.

In this method, single disk-shaped segments are processed at first into a cylindrical solid body by machining and subsequently provided with an axial slot by spark erosion. This generates disk-shaped structures per disk segment that obtain a defined angle of attack by subsequent plastic twisting around the longitudinal axis of the blade.

DE 100 53 664 A1 describes a mechanical kinetic vacuum pump with a rotor consisting of an Al alloy; for increasing the high-temperature strength and creep strength, it is proposed that the rotor material be a light metal alloy prepared by powder metallurgy whose main alloy component, apart from aluminum, is copper, further containing magnesium, manganese, zirconium and silver and optionally titanium.

By using a novel high strength and high high-temperature strength aluminum wrought alloy that has an unusually high elongation at break in a naturally aged state, it is now possible to employ the above mentioned less expensive shaping methods that had previously been reserved for low-strength to medium-strength Al alloys.

WO 2004/003244 A1 describes an Al—Cu—Mg—Mn alloy for the preparation of semifinished goods having high static and dynamic strength properties. Surprisingly, it could be found that the alloys described herein on the one hand have a particularly high high-temperature strength and on the other have so high a ductility in a naturally aged state that a low-cost rotor production by machining or thermal abrasion methods and reshaping (for example, twisting or bending) is possible.

Therefore, in a first embodiment, the invention relates to a process for preparing rotors or stators of a turbomolecular pump with rotor blades made of an Al—Cu—Mg—Mn wrought alloy, characterized in that the rotor blades are produced by machining or a thermal ablation method from individual disks or solid bodies, followed by generating a desired angle of attack by reshaping.

Thus, multistage, integral rotors or stators as well as rotors or stators assembled from individual stage segments are obtainable by means of the present invention. The rotors or stators have a low density and at the same time a high strength and ready processability.

It is particularly preferred according to the present invention for the wrought alloy employed to have the following composition:

-   from 0.3 to 0.7% by weight of silicon (Si); -   up to 0.15% by weight of iron (Fe); -   from 3.5 to 4.5% by weight of copper (Cu); -   from 0.1 to 0.5% by weight of manganese (Mn); -   from 0.3 to 0.8% by weight of magnesium (Mg); -   from 0.05 to 0.15% by weight of titanium (Ti); -   from 0.1 to 0.25% by weight of zirconium (Zr); -   from 0.3 to 0.7% by weight of silver (Ag); -   up to 0.05% by weight of others individually; -   up to 0.15% by weight of others in total; and -   aluminum (Al) as the % by weight balance.

As compared to other previously known alloys, the wrought alloys employed have a higher static and dynamic high-temperature strength and an improved creep strength while the mechanical properties at break are very good, and they are therefore particularly suitable for the rotors or stators of turbomolecular pumps according to the invention. In particular, the wrought alloy employed according to the invention has an elongation at break of at least 14%, especially from 17 to 20%, in a naturally aged state as determined in a tensile test according to DIN EN 10002.

The term “wrought alloy” within the meaning of the invention refers to a special treatment of the alloy employed according to the invention in which the cast structure is converted and “wrought” at an elevated temperature by, for example, extrusion, rolling or forging. The light metal is rendered more ductile thereby. Therefore, wrought alloys also allow further cold shaping operations, such as rolling, drawing or even forging (for example, cold forging).

From WO 2004/003244 A1, it is known per se that these properties are achieved, in particular, at a copper-to-magnesium ratio of from 5 to 9.5, especially at a ratio of from 6.3 to 9.3.

The copper content is preferably within a range of from 3.8 to 4.2% by weight, and the magnesium content is within a range of from 0.45 to 0.6% by weight. The copper content is clearly above the maximum solubility for copper in the presence of the claimed magnesium content, As a consequence, the proportion of insoluble copper-containing phases is very low, also in view of the remaining alloy and accompanying elements. This causes an improvement of the dynamic properties and the fracture toughness of the rotors prepared from such an alloy.

In contrast to aluminum alloys further known in the prior art, the silver content of the claimed wrought alloy is rather high with contents of from 0.3 to 0.7% by weight, preferably from 0.45 to 0.6% by weight. Due to the interaction with silicon (from 0.3 to 0.7% by weight, preferably from 0.4 to 0.6% by weight), a curing occurs through the same mechanisms as in silver-free Al—Cu—Mg alloys. However, for lower silicon contents, the course of the segregation is different because of the silver addition. Although the rotors prepared from such an alloy have good high-temperature strengths and creep strengths under cooler conditions, they do not meet the desired demands. Only silicon contents from 0.3% by weight suppress the otherwise typical change of the segregation behavior of Al—Cu—Mg—Ag alloys, so that higher strength values can be achieved for the Cu and Mg contents without a loss in high-temperature strengths and creep strengths.

The manganese content of the alloy employed is from 0.1 to 0.5% by weight, preferably from 0.2 to 0.4% by weight. For alloys with higher manganese contents, undesirable segregation processes resulting in a reduction in strength were found in a long-term high temperature exposure test. For this reason, the manganese content is limited to 0.5% by weight. In principle, however, manganese is an alloy component that is necessary for structural control.

For counterbalancing the reduced vacancies of manganese in view of structural control, the alloy contains zirconium in a proportion of from 0.10 to 0.25% by weight, especially from 0.14 to 0.2% by weight. The segregating zirconium aluminides are usually even more finely dispersed than manganese aluminides. Moreover, it has been found that the zirconium aluminides contribute to the thermal stability of the alloy.

For achieving a finer grain structure, from 0.05 to 0.15% by weight, preferably from 0.10 to 0.15% by weight, of titanium is added to the alloy. Conveniently, the titanium is added to the alloy in the form of an Al-5Ti-1B master alloy, whereby the alloy automatically contains boron. Finely dispersed insoluble titanium diborides are formed therefrom. These contribute to the thermal stability of the alloy.

The alloy may contain a maximum of 0.15% of iron, preferably 0.10% of iron, as an unavoidable impurity.

“Reshaping” within the meaning of the present invention includes, for example, twisting, bending, embossing and forging to give a desired angle of attack. The preparation of the desired angle of attack optionally also includes the preparation of a defined blade contour.

Alternatively, the steps of separating and reshaping may also be performed in one operation, for example, by coining.

The operations of this process according to the invention are known per se, but have been limited to low- and medium-strength aluminum alloys, because these are the only ones to have the required reshapability. By means of the present invention, however, this process can also be applied to the defined high-strength aluminum alloys.

Usually, the reshaping starts from disk-shaped blade stages in which individual blade segments are previously produced by radial separation. Separation methods within the meaning of the present invention include cutting methods, such as laser or water jet cutting, as well as eroding, machining, embossing or coining.

The combination of shaping by machining with methods of plastic reshaping, the production costs of the rotors or stators can be reduced.

EXAMPLES Example 1 Preparation of the Blades' Angle of Attack in Pump Rotors of a TMP by Reshaping

From a cylindrical solid body made of the alloy AA 2016 (see WO 2004/003244), disk-shaped segments concentrically superposed were prepared by machining in accordance with the desired number of pump stages. This produced a rotationally symmetric body consisting of superposed disk-shaped ribs interconnected in the hub region. The rib thickness corresponded to the later blade thickness. Now, every rib disk was slotted in axial direction to near the hub at regular intervals over the circumference to form individual blade segments.

During this, the material was in a “solution-annealed, quenched and naturally aged” state. In this state, it had a high reshapability.

Now, in the same state, the blade segments were twisted around their longitudinal axis. The twisting was effected with a fork-shaped grip arm that embraced a corresponding blade segment to near the blade foot and then performed a twisting movement around the blade's longitudinal axis to the desired angle of attack. This caused the blade segment to suffer a plastic reshaping in the zone near the blade foot. In this way, twisting angles or angles of attack of about 45° relative to the starting position were easily achieved without observing incipient cracks in the blade foot zone.

With conventional Al alloys having a high high-temperature strength, such angles cannot be achieved.

To achieve the strength required for the later operation, artificial ageing was performed to achieve the maximum strength according to state T6.

Example 2 Coining of Stator Disks

From Al sheets made of the material mentioned in Example 1, stator disks are prepared by coining as follows:

Semicircular ring segments are punched from Al sheets in thicknesses of from 0.5 to 1.0 mm. State of the sheets: “solution-annealed, quenched and aged naturally”.

These ring segments are now inserted in a coining die that will carve the blade contour by pressing the bottom die onto this coining die.

The leading edges of the radial-symmetrically arranged blade segments are punched out thereby whereas the angle of attack of the blades is produced by plastic reshaping within the coining die. The maximum reshaping occurs in the zone of transition from the free leading edge of the blade to the non-deformed sheet.

To achieve the strength required for the later operation, artificial ageing was performed to the maximum strength according to state T6. 

1. A process for preparing rotors or stators of a turbomolecular pump with rotor blades made of an Al—Cu—Mg—Mn wrought alloy, characterized in that the rotor blades are produced by machining or a thermal ablation method from individual disks or solid bodies, followed by generating a desired angle of attack by reshaping.
 2. The process according to claim 1, characterized by employing a wrought alloy having the following composition: from 0.3 to 0.7% by weight of silicon (Si); up to 0.15% by weight of iron (Fe); from 3.5 to 4.5% by weight of copper (Cu); from 0.1 to 0.5% by weight of manganese (Mn); from 0.3 to 0.8% by weight of magnesium (Mg); from 0.05 to 0.15% by weight of titanium (Ti); from 0.1 to 0.25% by weight of zirconium (Zr); from 0.3 to 0.7% by weight of silver (Ag); up to 0.05% by weight of others individually; up to 0.15% by weight of others in total; and aluminum (Al) as the % by weight balance.
 3. The process according to claim 1, characterized in that said rotors or stators are prepared by embossing, coining or twisting. 